As disclosed in the following patent documents 1, 2 and 3, electric power steering systems are provided conventionally as steering systems in vehicles.
Patent document 1: JP 2003-267247A (JP 3885621)
Patent document 2: JP 2003-319699A
Patent document 3: JP 2009-40149A
According to such conventional electric power steering systems, a voltage booster circuit is provided to boost (step up) a voltage of a battery and supply a motor drive apparatus with a boosted voltage. In the patent documents 2 and 3, the booster circuit is configured with a coil, switching elements, a backflow prevention element, a capacitor and a booster control circuit. The booster circuit in the patent document 1 is configured also in the similar manner.
The switching elements are specifically transistors such as n-channel MOS field effect transistors, each of which is referred to as FET.
In the booster circuit, induction energy is stored in the coil when the switching element is turned on, and the capacitor is charged with by a current when the switching element is turned off. The current flows in a direction, which is permitted by the backflow prevention element. The voltage is boosted by repetition of turning on and off of the switching element.
The booster control circuit controls the ON-duty ratio Don(%) of the switching element by feeding back the boosted voltage so that the boosted voltage attains a target voltage level. The ON-duty ratio Don is defined by the following equation.Don=ON-period/(ON-period+OFF-period)
A voltage step-down switch is connected in parallel to a diode, which is the backflow prevention element, for the following reason. When the switching element is in the OFF state, the voltage step-down switch is turned on so that the capacitor is charged by the current supplied from the coil. This current flows through the step-down switch of a small resistance without flowing through a diode of a large resistance. Thus, heat generation by the diode is reduced and as a result heat generation by the entire booster circuit is suppressed.
In some cases, the switching elements provided in the voltage booster circuit are subjected to a short-circuit failure. The switching elements are referred to as a step-up FET and a step-down FET The short-circuit failure of the step-up FET and the step-down FET is a short-circuit between a drain and a source in its OFF state. If the step-up FET is short-circuited, a large current flows from the coil in the OFF state and causes the step-up FET to generate heat. If the step-down FET is short-circuited, the voltage produced by the step-down FET is not boosted to a target level. Therefore, the ON-duty ratio is repetitively increased by the feedback control to promote the voltage boosting operation. As a result, the step-up FET is driven with almost the 100% ON-duty ratio, that is, driven to the full ON state.
It is proposed to detect possibility of short-circuit of a subject element during a system operation by persistently monitoring voltages developed at predetermined circuit points to which such a subject element is connected. For example, a voltage at a predetermined circuit point falls when a certain point in a circuit is turned on contrary to an original circuit design. If such a voltage fall is detected at the certain point, it is possible to determine that a subject element is likely to have the short-circuit failure. If the short-circuit failure is determined, a power supply relay is turned off to instantaneously shut off supply of the battery voltage to the booster circuit.
This fail-safe operation need be performed within a very short time, for example, in an order of 1/100 seconds. As the fail-safe operation time is set shorter, detection error may arise more often. For example, even when a voltage at a monitored point just accidentally falls in spite of no actual short-circuit failure of the subject element, the subject element is determined to be in failure erroneously and a power supply relay is turned off.
This determination error sometimes arises, when a power supply terminal for the booster circuit only momentarily cannot keep contact due to vibration or the like. Such a momentary disconnection is not influential to the operation of the booster circuit. It is not desirable to interrupt the power supply to the booster circuit too frequently by such a momentary disconnection.